The invention relates generally to asphalt pavement resurfacing equipment, as typified by a recycling apparatus, and more specifically to asphalt pavement heating devices which are utilized at one or more work stations located on the pavement recycling apparatus.
Asphalt pavement recycling apparatus, in general, as set forth in my copending application, Ser. No. 06/597,205, provide a means for resurfacing a roadway in which a substantial portion of the newly laid asphalt concrete is recycled material drawn form the existing roadbed. Existing pavement is viewed by the Federal Highway Department as a federal resource, and the use of this resource through recycling results in the conservation of asphaltic materials and usually in reduced paving costs.
The level of performance of a pavement recycling apparatus that employs some form of energy to heat the asphalt to a pliable state prior to working the surface is closely tied to the effectiveness of the heating device. The effectiveness of a heating device can be gauged on four criteria.
1. The time required to heat a specified depth of asphalt pavement to a predetermined temperature with an acceptable level of surface degradation. Asphalt concrete compositions become pliable at 250.degree. F., and so this is a commonly used minimum target temperature. PA1 2. The cost in terms of energy of heating the asphalt. PA1 3. The ability of the heating device to withstand the rigors associated with heavy equipment related road construction. PA1 4. The environmental impact incidental to the operation of the heating device.
The instant invention distinguishes itself in each of the four criteria.
Billowous, black fuming smoke has historically been associated with direct flame impingement asphalt cement heaters. Heretofore it has been thought necessary that to prevent degradation indirect, "non-oxidizing" heaters had to be used, and in response a variety of methods have been developed. Most have used some form of radiant energy, either infrared (U.S. Pat. No. 3,865,098 Cutler) or Microwave (U.S. Pat. No. 4,319,856 Jeppson), while others used a non-flammible medium such as steam (U.S. Pat. No. 4,261,669 Edo). A gas fired ceramic heater can operate at temperatures as high as 2100.degree. F., while most operate in the range of 1400.degree.-1600.degree. F. Gas fired infrared heaters will degrade the asphalt and smoke if the pavement surface reaches a temperature in excess of 500.degree. F. in the presence of air.
Infra-red heating generally reduces the level of oxidation as compared to most open flame burners, however, current asphalt pavement resurfacing equipment employing this energy source still generates significant, undesirous amounts of black smoke. This suggests that degradation is still occurring, and the mechanism is probably via oxidation. Gas fired infrared ceramic heaters are relatively fragile in that the ceramic target element is friable and is easily subject to damage. To protect the ceramic element a metal guard screen is frequently inserted between the heater and the pavement surface. This improvement in durability, however, is somewhat off-set by the reduction in heat efficiency. The radiant energy collected by the screen is re-irradiated, however, at a lower temperature. Some heat of course will also be lost to the surroundings by conduction.
The art of heating high molecular weight organic materials is well known to those in the petroleum industry. Asphaltic materials are routinely refined using thermal cracking, and aluminum silicate salts, such as found in gravel, are employed as catalysis. It is imperative that the refining system be non-oxidative or else degradation occurs. The cruciality of maintaining a non-oxidative environment is not reflected in the literature reading on the art of heating asphalt pavement. Apparently some assumptions were made early on as to the reason why open flame heaters were impractical, and this resulted in emphasis on other methods. Open flame heaters are inherently simpler, cheaper and can be made more efficient than radiant heaters.
I have found that when using the heating device as disclosed herein that asphalt concrete can very quickly and efficiently be heated to a pliable state using direct flame impingement without either degrading the pavement or generatin the reported fuming, black smoke. The heating device operates so cleanly that the exhaust is virtually colorless.